The aim of the proposed research is to describe the neural, general metabolic, and endocrine factors responsible for the regulation of body weight and body fat. A research plan comprised of three parts is proposed. The first part is devoted principally to proposals for extending and clarifying earlier observations concerning the role of the lateral hypothalamus in setting the level of maintained body weight and body fat. The second section, which represents the main thrust of the proposal, presents a plan for studying and comparing the metabolic features of animals chronically maintaining their body weight at reduced, normal, or elevated levels. Ventromedial and lateral hypothalamically lesioned animals, and the genetically obese Zucker rat, will be used in these studies. In the final section, proposals are made to study the endocrine factors associated with regulation of body weight and body fat at different levels in both hypothalamically-lesioned and genetically obese preparations.